1. Field of the Invention
This invention relates to a method of primary recrystallization annealing of cold rolled steel strip that is raw material for grain-oriented electrical steel strip for use in power transformer cores and the like.
2. Description of the Prior Art
All grain-oriented electrical steel strip currently being used in practical applications invariably has a steep crystallographic texture formed by secondary recrystallization. This secondary recrystallization texture is controlled to have a grain orientation which ensures that when the grain-oriented electrical steel strip is used for the core of a transformer or other such electric device, the device will exhibit optimum performance and efficiency. The performance of the electric device improves in proportion with increasing steepness of the secondary recrystallization texture. Generally speaking, the steepness of the secondary recrystallization texture is strongly reflected in the magnetic flux density (B.sub.8 value: degree of magnetization in an 800 AT/m magnetic field) of the grain-oriented electrical steel strip. Those concerned with the development and production of grain-oriented electrical steel strip are therefore interested more than anything else in how secondary recrystallization can be made to occur optimally in the process of manufacturing such steel strip.
Secondary recrystallization is a phenomenon involving abnormal growth of primary recrystallization grains with extremely high orientation selectivity.
It is well known that the strength of this orientation selectivity, which governs the magnetic flux density of grain-oriented electrical steel strip, is dependent on such factors as the crystallographic texture of the primary recrystallization grain structure, the average grain diameter and the inhibitor strength (the resistance to grain boundary movement imparted by precipitates and grain boundary segregation elements). Notwithstanding, no method has so far been disclosed for measuring primary factors governing the magnetic flux density of grain-oriented electrical steel strip and then using the result of the measurement for ensuring optimum formation of the primary recrystallization structure. Moreover, in the industrial production of grain-oriented electrical steel strip there is sometimes experienced large-scale occurrence of poor secondary recrystallization (to the extent that the products have to be scrapped). Although this happens only rarely, it is a very serious problem because of the manner in which grain-oriented electrical steel strip is produced. Specifically the finish annealing step of the grain-oriented electrical steel strip is a batch annealing process in which a large number of strip coils of the same type are simultaneously treated and requires between 150 and 200 hours. Therefore, when poor secondary recrystallization is discovered after finish annealing in even a small number of the strip coils, there is high probability that a high percentage of the remaining strip coils also suffer inferior secondary recrystallization. Unfortunately, it takes a long time to determine the cause of inferior secondary recrystallization and establish the required countermeasures. Once inferior secondary recrystallization arises, therefore, the scale of the loss it produces is likely to be large.
Product-to-product variation in magnetic flux density is observed even in normal grain-oriented electrical steel strip produced under ordinary conditions.
Such variation in magnetic flux density is observed not only among products from different steel lots (despite the lack of any difference in composition), but also among strip coils from the same steel lot and even among different parts of one and the same strip coil. It is thus these variations that impede the stabilization of the magnetic flux density of grain-oriented electrical steel strip at a high level. The cause lies in the fact that the secondary recrystallization behavior is highly sensitive to subtle changes in the various processing conditions involved in the production of grain-oriented electrical steel strip.
If the cause of the inferior secondary recrystallization and the variation in magnetic flux density is present at point prior to the finish annealing step, its effect can also be expected to be manifested with respect to the primary recrystallization grain diameter and other factors governing the magnetic flux density. Therefore, if it should be possible during the primary recrystallization annealing step to conduct online measurement of the primary recrystallization grain diameter and other factors governing the magnetic flux density, this would be highly effective for preventing the large-scale occurrence of poor secondary recrystallization. Moreover, if it should be further possible to eliminate the factors adversely affecting the product magnetic flux density by adjusting the primary recrystallization annealing conditions, this would make it possible to stabilize the magnetic flux density of the grain-oriented electrical steel strip at a high level.